Compact ultra fast laser

ABSTRACT

The solid state laser comprises a laser gain medium ( 1 ), pumping means for pumping the laser gain medium, and a laser cavity having a first end ( 3 ) and a second end ( 17 ), wherein the laser gain medium is at, or in the vicinity of, said first end ( 3 ) of said cavity. A semiconductor saturable absorber mirror (SESAM) can be placed at the second end ( 17 ) of the cavity. The laser gain medium can comprise at least one face for receiving pumping energy from the pumping means, the face being made reflective at a laser frequency of the laser, so that it can form the first end of the laser cavity. The resulting setup used for generating femtosecond laser pulses.

FIELD OF THE INVENTION

[0001] This invention relates to compact solid state lasers.

BACKGROUND OF THE INVENTION

[0002] Femtosecond lasers are usually more complicated than other lasersemitting continuous-wave, Q-switched, or picosecond radiation. Onereason for this is that femtosecond generation requires laser materialswith a spectrally broad emission band, in comparison for example to thewell-known laser material Nd:YAG, leaving a limited number of lasermaterials suitable for femtosecond generation. Additionally, femtosecondlasers need some group velocity dispersion compensation, which usuallyrequires additional intra cavity elements, such as a prism pair, therebyadding complexity to the system. An example of a femtosecond laser isthe green-pumped Ti:sapphire laser. More compactness is obtained bydirectly diode pumping suitable laser materials, such as Nd:glass,Cr:LiSAF, Yb:glass, etc (see for example in D. Kopf, et al.,“Diode-pumped modelocked Nd:glass lasers using an A-FPSA”, OpticsLetters, vol. 20, pp. 1169-1171, 1995; D. Kopf, et al., “Diode-pumped100-fs passively modelocked Cr:LiSAF using an A-FPSA”, Optics Letters,vol. 19, pp. 2143-2145, 1994; C. Hönninger, et al., “Femtosecond Yb:YAGlaser using semiconductor saturable absorbers”, Optics Letters, vol. 20,pp. 2402-2405, 1995). These laser systems, however, are not perfectlycompact in the sense that they usually use two laser diodes as pumpsources that are imaged into the laser crystal using imaging optics. Thelatter are relatively large in size and could still be made considerablymore compact. Furthermore, the resonator comprises two arms that have tobe aligned accurately with respect to each other and with respect to thepump beam, respectively, resulting in a number of high-accuracyadjustments to be performed.

[0003] A setup of this type is known from U.S. Pat. No. 5,987,049. Thispatent discloses a pulsed solid-state laser comprising a two-armedoptical resonator with a solid-state laser medium and a semiconductorsaturable absorber mirror device (SESAM) placed inside. A prism pair isincorporated for dispersion-compensating purposes. The achievablecompactness of the setup is limited due to the positions of the SESAMand the prism pair at each end of the cavity arms.

[0004] Quite commonly, focusing lenses with a focal length of 75 mm orlonger are used to focus the pump light into the laser crystal throughone of the curved cavity mirrors, following a delta-type laser cavityscheme. Such a cavity scheme essentially does not allow forstraight-forward size reduction of the pump optics. Another approach(see for example S. Tsuda, et al., “Low-loss intracavity AlAs/AlGaAssaturable Bragg reflector for femtosecond mode locking in solid-statelasers”, Optics Letters, vol. 20, pp. 1406-1408, 1995) places the lasermedium at the end of the laser cavity, thereby allowing for more compactpump focusing optics with a potentially shorter working distance andreducing the number of adjustments required. However, since one cavityend is taken by the laser medium, both the semiconductor element(semiconductor saturable absorber mirror, SESAM) and the prism sequencefor dispersion compensation need to be placed toward the other end ofthe laser resonator. Since the spot size on the SESAM needs to be smallenough for saturation in that setup, the focusing mirror towards thatcavity end does not leave enough room for a prism pair to compensate forthe group velocity dispersion. However a total of four prisms had to beimplemented for that purpose.

SUMMARY OF THE INVENTION

[0005] The invention relates to compact solid state lasers. The lasermedium is positioned at or close to one end of the laser cavity andpumped by at least one pump source or laser diode. The pumping can bedone by one or two laser diodes including imaging optics of compact size(10 cm or less), respectively, due to the arrangement of the cavity endand pumping optics, and is suitable for achieving reasonable gain evenfrom low-gain laser materials. For femtosecond operation, the laserresonator is laid out such that both a semiconductor saturable absorbermirror and a prism pair are located toward the other end of the cavity,and the laser mode on the SESAM and the prism sequence length fulfillthe requirements that have to be met for stable femtosecond generation.It is another object of the invention to provide a semiconductorsaturable absorber mirror (SESAM) having a structure which comprises aplurality of alternating gallium arsenide (GaAs) and aluminum arsenide(AlAs) or Aluminum gallium arsenide (AlGaAs) layers, each layer havingan optical thickness corresponding substantially to one quarterwavelength, a gallium arsenide (GaAs) substrate at a first face of saidplurality of alternating layers, a gallium arsenide (GaAs) or AlGaAsstructure integrating an absorber layer at a second face of saidplurality of alternating layers, and plurality of dielectric layers at aface of said gallium arsenide (GaAs) opposite the one in contact withsaid second face, whereby the overall structure shows resonant behavior.Such a SESAM may be implemented into a solid state laser as describedabove. It is a further object of the invention to provide a specialsetup for a solid state laser, wherein the laser comprises a laser gainmedium, pumping means for pumping said laser gain medium, a laser cavitywith a semiconductor saturable absorber mirror (SESAM) at one end ofsaid cavity, and wherein said cavity contains a prism pair followed by atelescope.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention and its advantages shall become more apparent fromreading the following description of the preferred embodiments, givenpurely by way of non-limiting illustrative examples with reference tothe appended drawings, in which:

[0007]FIG. 1 is a schematic representation of a laser gain setupaccording to a preferred embodiment of the invention;

[0008]FIG. 2 is a schematic representation of an unfolded propagation ofthe laser mode cavity of a femtosecond cavity;

[0009]FIG. 3 is a schematic representation of an implementation of thecavity of FIG. 2 forming a small-size setup;

[0010]FIGS. 4a and 4 b are schematic representations of implementationsof the cavity of FIG. 2 with a relatively larger prism sequence,followed by an intracavity telescope and the cavity end;

[0011]FIG. 5 shows an example of a semiconductor saturable absorberstructure which can be used in combination with prism sequences;

[0012]FIGS. 6a through 6 d show various embodiments of a second pumpingsource;

[0013]FIGS. 7a through 7 d show various embodiments of the secondpumping source in combination with a special orientation of aBrewster-cut gain medium to form a compact setup; and

[0014]FIG. 8 is an example of a setup with a plurality of dispersivemirror structures forming a folded cavity

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The general setup of a compact, ultra-fast laser according to apreferred embodiment of the invention shall be described with referenceto FIG. 1. The gain section of the laser setup comprises a laser gainmedium 1 which is located in the vicinity of a first end of a lasercavity (see laser cavity mode axis 2). The laser gain medium 1 can evenbe the laser cavity end itself if one side 3 of the laser material iscoated for reflectivity at the laser wavelength. A flat-Brewster-cutlaser medium may be used, where the flat side is coated for reflectivityat the laser wavelength and for high transmission at the wavelength ofthe pump laser diode 4 used in the setup. The laser diode beam ispreferably collimated in the (vertical) fast-divergent axis by means ofa cylindrical micro lens attached close to the laser diode 4 so that thepump beam 5 diverges at a reduced vertical divergence angle. The pumplaser diode 4 can be for example a 100 micron wide laser diode emittingat a power of 1 or more Wafts at a wavelength of 800 nm. It serves topump a laser medium such as Nd:glass. A collimating lens 6 and focusinglens 6′ are used to re-image the pump beam into the laser medium 1.Imaging elements including the microlens, and lenses 6 and 6′ may bereplaced by any imaging optics of similar compactness and imagingproperties. Because of the potentially short working distance betweenlens 6′ and the laser medium 1, the pump elements 4,6,6′ can cover asshort a distance, on the order of 10 cm or less.

[0016] The setup uses a second pump source comprising a laser diode 7,collimating lens 8, prism 9, focusing lens 10, and dichroic mirror 11.The pump beam of laser diode 7 is first collimated with lens 8 and thenenters prism 9. When the beam emerges from the prism 9, it has beenexpanded in the tangential plane, as indicated in FIG. 1. This resultsin a smaller spot in air after focusing lens 10. One or the other ofthese laser diodes, or both combined, may produce a pump intensity of 10kW per square centimeter or more. When entering the laser medium 1through the Brewster face, however, the spot will be expanded again dueto the Brewster face refraction. Therefore the prism 9 is used topre-compensate the expansion due to the Brewster face, which results insimilar spot sizes within the laser medium 1 from both pump sources.Additionally, the prism 9 is used to compensate for the beam axis angledue to the Brewster face of the laser medium. The pump source comprisinglaser diode 7, lens 8, prism 9, and lens 10 can have a degree ofcompactness similar to that of the first pump source, assuming thatdichroic mirror 11 is placed close enough to the laser medium 1,reducing the working distance between the lens 10 and the laser medium.The dichroic mirror 11 is highly transmissive for the pump wavelength oflaser diode 7 and highly reflective for the laser wavelength. In thisway, the resonator mode 2 is directed from the laser medium 1 towards acurved cavity mirror 12 and some further plane folding mirrors 13 and13′, etc., for example. When the focus spot of the pump sources 4 and 7is chosen to be located within the laser medium 1, this pump arrangementis suitable for pumping low-gain laser materials such as Nd:glass,Cr:LiSAF, Yb:glass, Yb:YAG, Yb:KGW, etc (low-gain meaning less gain thanNd:YAG). This pump arrangement can therefore be used for pumping broademission band laser materials suitable for femtosecond generation. Itmay however also be used for pumping any solid state laser material forother purposes including continuous wave, Q-switched, or picosecondoperation.

[0017] For a femtosecond laser setup, above setup can be combined withthe laser mode shown in FIG. 2, which illustrates an example of anunfolded propagation of the laser mode throughout a possible femtosecondcavity. The lenses indicate curved cavity mirrors that refocus thecavity mode. Laser medium 1 in the vicinity of one cavity end 3′ has amode radius on the order of 30×45 um (microns). The cavity end 3′ may bea mirror with characteristic features similar to those of the coatedside 3 of the laser material in FIG. 1. Curved mirror 12 (whose radiusof curvature is for example 200 mm) is located some 120 mm away from thelaser medium 1, and therefore re-images the cavity mode into a waist 14.The cavity mode then further diverge to a spot size that is on the orderof 2-3 mm in diameter at another cavity mirror 15 (whose radius ofcurvature is for example 600 mm) after a distance 16 of around 1400 mm.The relatively large mode diameter at cavity mirror 15 results in asmall mode diameter 16 a at the laser cavity end which contains a SESAM(semiconductor saturable absorber mirror) 17. An example of a design fora suitable SESAM is given in D. Kopf, et al., “Diode-pumped femtosecondsolid state lasers based on semiconductor saturable absorbers”, SPIEProceedings, “Generation, Amplification and Measurement of UltrashortLaser Pulses III”, Jan. 28-30, 1996, San Jose, Calif., The InternationalSociety for Optical Engineering). This laser cavity has a large workingdistance of around 400 mm between element 15 and 17 such that it cancontain a group delay component such as a prism pair 18,18′ (shownschematically, see also FIG. 4b) consisting of two SF10 Brewster prismsthat are separated by some 350 mm for sufficient group velocitydispersion compensation. The prism pair 18, 18′ as group delay componentand the SESAM 17 are combined to form a beam influencing system B thatis located between the laser medium 1 and an end of the cavity which isidentical with the SESAM 17 in this special example. Other suitablegroup delay components are dispersive mirror structures such as aGires-Tournois-Interferometer or mulitlayer dielectric mirrors. Anexample for the use of such devices is disclosed in R. Paschotta, etal., “Double-chirped semiconductor mirror for dispersion compensation infemtosecond laser”, Appl. Phys. Lett., Vol. 75, No. 15, October 1999,2166-2168.

[0018] The cavity of FIG. 2 can be folded with plane highly reflectivemirrors or dispersive mirror structures at any location as required tofit the setup into small boxes. One example of a final small-size setupis shown in FIG. 3. Here the surface 3 of the laser medium 1 is madepartially transmissive for the laser wavelength such that a fraction ofthe intracavity power is outcoupled and furthermore separated from theincident pump beam by dichroic mirror 3 b, resulting in laser outputbeam 3 c. Prism sequences that are considerably longer than those inabove setup can be achieved at the expense of a larger spot size at theend of the prism sequence.

[0019]FIGS. 4a and 4 b illustrate such examples of prism sequences. Forsuch longer prism sequences 19, for example 500-1000 mm long or longer,the spot size 20 at the SESAM could be too large for achievingsaturation at femtosecond operation as required for stable ultra fastperformance. To solve this problem, it can be useful to extend thecavity by a telescope 21. In this way, the mode size reduces accordingto the telescope factor to a mode size 21′ (FIG. 4a), where the SESAM ispositioned. Simultaneously, the parallelism between two dispersed beams22 and 22″ is preserved after the telescope, and corresponding beams 23and 23′ (FIG. 4a) are perpendicular to the end mirror (which is theSESAM) 24 as required for the lasing condition and for obtainingnegative group velocity dispersion from the prism sequence 19. Prismsequences of considerable length can also be used in combination with aspecial SESAM structure such that saturation is obtained at lower energydensities for stable ultra fast laser operation.

[0020]FIG. 5 shows an example of such a semiconductor saturable absorberstructure, representing the layers along the surface normal to itssurface. Firstly, 30 pairs of layers of alternating gallium arsenide(GaAs) and aluminum arsenide (AlAs) layers 43 each with an opticalthickness corresponding to a quarter wavelength are applied onto agallium arsenide (GaAs) substrate 48. This can be achieved by means ofgrowth process using molecular beam epitaxy (MBE). However, other knownepitaxy processes and usual in this field are also suitable. TheGaAs/AlAs pairs of layers are transparent for the laser wavelength of1064 nm and result, in the example of FIG. 5, in a Bragg mirrorlikecoating structure with a high reflection factor close to 100% with awavelength of 1064 nm if the thickness of GaAs is selected at approx.72.3 nm and that of AlAs at approx. 88 nm, each corresponding to aboutan optical quarter wavelength. Then, a further GaAs layer 44 integratingan approx. 10 nm thin absorber layer of indium gallium arsenide (InGaAs)material is assembled onto this standard GaAs/AlAs Bragg mirrorstructure. The optical total thickness of this GaAs layer withintegrated absorber layer 47 corresponds to half a wavelength, that isthe physical film thickness is approx. 145 nm. The indium content of theabsorber layer 47 is determined so that an absorption is obtained at thelaser wavelength of 1064 nm, that is the band-edge is approx. 1064 nm ora few 10 nm higher than the laser wavelength, e.g. at 1064-1084 nm. Thiscorresponds to an indium content of about 25 percent. With higherintensity and pulse energy density, a saturation of the absorption ofthis absorber layer 47 occurs, i.e. it is lower. In the case ofparticularly thin layers of less than 20 nm thickness, by additionallyfinely adjusting the indium content, the exciton peak near the bandedge, generated by the exciton absorption behaviour of thin layers to bequantizised, can be adjusted exactly to the laser wavelength, resultingagain in an even more pronounced saturable absorption at thatwavelength. Finally, another three or more pairs of dielectric layerstransparent for the layer wavelength are applied, beginning with thatlayer 45 having a higher index of refraction n=2.02 and continuing withthat layer 46 having a lower index of refraction of 1.449 at awavelength of 1064 nm. The process of electron beam coating, widespreadin the optical coating field, is suitable to achieve this. Other opticalcoating processes, such as for example ion beam sputtering, are alsosuitable and can have the advantage of resulting in lower losses. Asoptical layer materials, those with an index of refraction of 1.449 and2.02 at a wavelength of 1064 nm were used. However, a large number ofother materials can be used as long as adhesion to GaAs and transparencyat the laser wavelength are ensured. Because the three or more finaldielectric pairs have a reversed order in terms of their index ofrefraction, with respect to the order of the refractive indexes of thelayers underneath, the structure is at resonance. By virtue of theresonant saturable absorber mirror structure, this device has asaturation fluence which can be on the order of a few microjoules persquare centimeter (depending on the number of dielectric top layers),which is considerably lower than those of existing SESAMs, and cantherefore be well suited for femtosecond or pulsed laser generation fromsetups where the laser mode on the saturable absorber device is usuallytoo large for saturation. Thanks to the resonant structure, one singleor a low number of single thin saturable absorber layers introduce anincreased saturable absorption for the overall device in comparison tothose structures which do not use a resonant structure. When thesaturable absorber layers introduce strain due to a lattice mismatch(which is the case for Indium Gallium Arsenide within GaAs), thisstructure helps reduce strain without reducing the saturable absorptioneffect for the overall device, resulting in less material defects and inimproved long-term properties of the device.

[0021]FIGS. 6a to 6 d show various special embodiments of the pumpingmeans. FIG. 6a is schematic setup, which is almost identical to thesetup in FIG. 1, comprising pumping means with two different pumpingsources 4 and 7, e.g. two semiconductor lasers, located at both ends ofthe laser medium 1. In FIG. 6b the second pumping source is replaced bya reflective element 7′ and a dichroic mirror 11 substitutes the prism9. The pump light of the first pumping source 4 passes through the lasermedium and is focused onto the reflective element 7′ by a combination ofa second collimating lens 8 and a second focusing lens 10. This setupcouples the first focus of the first pumping source 4 with the secondfocus of the reflected pump beam as a second pumping source. The pumpbeam of the first pumping source 4 is reflected into itself. A movementof the first pumping source and therefore of the first focus causes anaccording movement of the second focus on the same order. Therefore thetwo foci remain aligned without a necessity for an adjustment. FIG. 6cshows the setup of FIG. 6b with a reflection of the first pump beam bythe reflective element 7′ after a collimation. A movement of the firstpumping source leads to a movement of the second focus on the same orderbut in the opposite direction. Therefore the relative displacement ofboth foci is twice the initial displacement of the first focus. In FIG.6d the reflective element 7′ substitutes the dichroic mirror to form avery compact setup without any second pumping source.

[0022]FIGS. 7a to 7 d are schematic setups with a special orientation ofa Brewster-cut laser medium 1. The laser cavity comprises a sequence offolding mirrors defining a folded part F. The laser medium 1 shows aBrewster-cut design with a Brewster face 3′. This Brewster face 3′ isorientated looking away from the folded part F (“outwards orientation”)to allow a very compact cavity setup due to the flat angle of the beamreflected by the prism 9′. This angle is measured with respect to theaxis of the laser medium 1. The different FIGS. 7a to 7 d show thedifferent setups of pumping means as disclosed in FIGS. 6a to 6 d. Asshown in these examples the special orientation of the Brewster-face 3′is not limited to the use of a beam influencing system.

[0023]FIG. 8 is a schematic view of a multiple folded compact lasercavity with an outwards orientated Brewster-face 3′. The optical pathinside the folded part F of the laser cavity is defined by plurality ofdispersive mirror structures 18″ with negative group delay dispersion.These mirror structures combine the features of group delay dispersioncompensation and reflection to replace both, prisms and folding mirrors.The beam influencing system now comprises the SESAM 17 and thedispersive mirror structures 18″.

[0024] While there has been described herein the principles of theinvention, it is to be clearly understood to those skilled in the artthat this description is made only by way of example and not as alimitation to the scope of the invention. Accordingly, it is intended,by the appended claims, to cover all modifications which fall within thespirit and scope of the invention.

[0025] The application of pump setups as shown in FIGS. 6a to 6 d andthe special orientation of a Brewster-face as shown in FIGS. 7a to 7 dis not restricted to their combined use or in combination with a beaminfluencing system. Although the combination of these features allows avery compact laser design these setups are also applicable to a varietyof different laser devices.

1. A solid state laser comprising: a laser gain medium (1), pumpingmeans for pumping said laser gain medium (1), a laser cavity, and a beaminfluencing system (B) with a semiconductor saturable absorber mirror(17), wherein said beam influencing system (B) is located between saidlaser gain medium (1) and a first end of said laser cavity and said beaminfluencing system (B) comprises at least two prisms (18, 18′) or atelescope.
 2. The solid state laser according to claim 1, wherein saidbeam influencing system (B) comprises said two prisms (18, 18′); saidsemiconductor saturable absorber mirror (17) is located at one end ofsaid laser cavity and said laser cavity comprises a telescope, whereinsaid prism pair (18, 18′) is followed by said telescope.
 3. The solidstate laser according to claim 1 or 2, wherein a laser mode within saidbeam influencing system (B) is convergent in a sense that at least onediameter of the cross section of said laser mode is decreasing towardssaid semiconductor saturable absorber mirror (17).
 4. The solid statelaser according to claim 1, 2 or 3, wherein said beam influencing system(B) comprises at least a dispersive mirror structure (18″), such as aGires-Toumois Interferometer or a multiplayer dielectric mirror.
 5. Thesolid state laser according to one of the preceding claims, wherein saidbeam influencing system (B) is at, or in the vicinity of, said firstend.
 6. The solid state laser according to one of the preceding claims,wherein said laser gain medium (1) is at, or in the vicinity of, asecond end of said laser cavity.
 7. The solid state laser according toone of the preceding claims, wherein said laser cavity is folded byhighly reflective mirror means (13, 13′) and/or by at least onedispersive mirror structure (18″) for integration in a compact setupapplication.
 8. The solid state laser according to claim 7, wherein aidlaser gain medium (1) comprises a Brewster face (3′) and said mirrormeans (13, 13′) and/or said at least one mirror structure (18″) define afolded part of said cavity and said gain medium (1) is orientated withsaid Brewster face (3′) looking away from said folded part.
 9. The solidstate laser according to one of the preceding claims, wherein said lasergain medium (1) comprises at least a first face (3) for receivingpumping energy from said pumping means, said first face (3) being madereflective at a laser frequency of said laser, whereby said laser gainmedium (1) forms said second end.
 10. The solid state laser according toclaim 9, wherein said first face (3) is a flat face of aflat-Brewster-cut laser gain medium (1).
 11. The solid state laseraccording to claim 9 or 10, wherein said laser gain medium (1) comprisesa second face and said pumping means comprise: a first part with a firstpumping source (4) and a second part, said first pumping source (4)producing a first pumping beam at said first face (3) and said secondpart producing a second pumping beam at said second face.
 12. The solidstate laser according to claim 11, wherein the second part comprises asecond pumping source (7) or a reflective element (7′), said reflectiveelement (7′) reflecting said first pumping beam after a passage of saidlaser gain medium (1) as said second pumping beam at said second face.13. The solid state laser according to claim 12, wherein after saidpassage of said laser gain medium (1) said first beam is collimated oris focused on said reflective element (7′).
 14. The solid state laseraccording to claims 11, 12 or 13, wherein said second part comprises asecond optical path from said second pumping source (7) or saidreflective element (7′) to said laser gain medium (1), said secondoptical path comprising a prism element (9) and a dichroic mirror (11)or a prism element (9′) with a reflecting face.
 15. The solid statelaser according to claim 14, wherein said second optical path comprisesa second collimating lens (8) and a second focusing lens (10).
 16. Thesolid state laser according to the one of the claims 9 through 15,comprising a first collimating lens (6) and a first focusing lens (6′)to re-image said first pumping beam into said laser gain medium (1),with a working distance between said first focusing lens (6′) and saidfirst face (3) less than 50 mm
 17. The solid state laser according toone of the claims 9 through 16, wherein a first optical path from a saidfirst pumping source (4) to said laser gain medium (1) is on the orderof 10 centimeters or less.
 18. The solid state laser according to one ofthe preceding claims, wherein at least one beam spot produced by saidpumping means is located within said laser gain medium (1).
 19. Thesolid state laser according to one of the preceding claims, wherein saidlaser cavity is a femtosecond cavity.
 20. The solid state laseraccording to one of the preceding claims, wherein said laser gain medium(1) has a composition taken from the group comprising: Nd:glass,Cr:LiSAF, Yb:glass, Yb:YAG, Yb:KGW.
 21. The solid state laser accordingto one of the preceding claims, wherein said laser gain medium (1) has acomposition having a gain equal to or smaller than a gain obtained fromthe composition Nd:YAG or Yb:YAG, with said gain to be determined as theproduct of the stimulated emission cross section and the upper laserlevel life time.
 22. The solid state laser according to one of thepreceding claims, wherein said laser gain medium (1) is a broad emissionband laser material suitable for femtosecond laser generation.
 23. Thesolid state laser according to claim 6, wherein said laser gain medium(1) has a mode radius on the order of 30 microns×45 microns.
 24. Thesolid state laser according to one of the preceding claims, wherein thepumping means have a pump intensity equal to or greater than 10 kW persquare centimeter.
 25. The solid state laser according to one of thepreceding claims, further comprising a first curved mirror (12) at anoutput of said laser gain medium (1) arranged to re-image and cavitymode into a waist (14).
 26. The solid state laser according to one ofthe preceding claims, further comprising a second curved mirror (15)between said waist (14) and said first end.
 27. The solid state laseraccording to claim 26, wherein a distance between said second curvedmirror (15) and said first end is on the order of 40 centimeters orlonger.
 28. The solid state laser according to claim 26 or 27, whereinsaid beam influencing system (B) is located between said second curvedmirror (15) and said first end.
 29. The solid state laser according toone of the preceding claims, wherein said semiconductor saturableabsorber mirror (17) is a layered structure comprising: a plurality ofalternating layers (43) of gallium arsenide and aluminum arsenide oraluminum gallium arsenide, each layer having a thickness correspondingsubstantially to one quarter wavelength, a substrate (48) of galliumarsenide at a first layer face of said plurality of alternating layers(43), a structure (44) of gallium arsenide of aluminum gallium arsenideintegrating an absorber layer (47) at a second layer face of saidplurality of alternating layers (43), and a plurality of dielectriclayers (45, 46) at a face of said structure (44) opposite the one incontact with said second face, whereby the overall structure showsresonant behaviour.
 30. Use of the laser according to one of the claims1 through 29 for generating femtosecond laser pulses.
 31. Use of thelaser according to one of the claims 1 through 30 for continuous wave orQ-switched laser operation.
 32. A semiconductor saturable absorbermirror (17) for a solid-state laser, particularly for a solid statelaser according to one of the claims 1 through 29, said semiconductorsaturable absorber mirror (17) having a layered structure comprising: aplurality of alternating layers (43) of gallium arsenide and aluminumarsenide or aluminum gallium arsenide, each layer having a thicknesscorresponding substantially to one quarter wavelength, a substrate (48)of gallium arsenide at a first layer face of said plurality ofalternating layers (43), a structure (44) of gallium arsenide oraluminum gallium arsenide integrating an absorber layer (47) at a secondlayer face of said plurality of alternating layers (43), and a pluralityof dielectric layers (45, 46) at a face of said structure (44) oppositethe one in contact with said second face, wherein said dielectric layers(45, 46) have a reversed order in terms of their index of refraction,with respect to the order of the refractive indexes of the layersunderneath, thereby forming a resonant structure, whereby the overallstructure shows resonant behaviour.
 33. The semiconductor saturableabsorber mirror (17) according to claim 32, wherein said plurality ofalternating layers (43) is on the order of 30 in number.
 34. Thesemiconductor saturable absorber mirror (17) according to claims 32 or33, wherein each of said plurality of alternating layers (43) has athickness respectively of approximately 72.3 nanometers andapproximately 88 nanometers.
 35. The semiconductor saturable absorbermirror (17) according to one of claims 32 through 34, wherein a totaloptical thickness of said structure (44) corresponds to half awavelength.
 36. The semiconductor saturable absorber mirror (17)according to one of the claims 32 through 35, wherein said dielectriclayers (45, 46) are three or more in number.
 38. A solid state lasercomprising: a laser gain medium (1), pumping means for pumping saidlaser gain medium (1), a semiconductor saturable absorber mirror (17)located towards a first end of said cavity, and a first curved mirror(12) at an output of said laser gain medium (1) arranged to re-image acavity mode into a waist (14).
 39. The solid state laser according toclaim 38, wherein said semiconductor saturable absorber mirror (17)comprising: a plurality of alternating layers (43) of gallium arsenideand aluminum arsenide or aluminum gallium arsenide, each layer having athickness corresponding substantially to one quarter wavelength, asubstrate (48) of gallium arsenide at a first layer face of saidplurality of alternating layers (43), a structure (44) of galliumarsenide or aluminum gallium arsenide integrating an absorber layer (47)at a second layer face of said plurality of alternating layers (43), anda plurality of dielectric layers (45, 46) at a face of said structure(44) opposite the one in contact with said second face, whereby theoverall structure shows resonant behaviour.
 40. The solid state laseraccording to claim 38 or 39, wherein said laser cavity is a femtosecondcavity.
 41. The solid state laser according to claim 38, 39 or 40,wherein said laser gain medium (1) has a composition taken from thegroup comprising: Nd:glass, Cr:LiSAF, Yb:glass, Yb:YAG, Yb:KGW.
 42. Thesolid state laser according to one of the claims 38 through 41, whereinsaid laser gain medium (1) has a composition having a gain smaller thana gain obtained from the composition Nd:YAG or Yb:YAG, with said gain tobe determined as the product of the stimulated emission cross sectionand the upper laser level life time.
 43. The solid state laser accordingto one of the claims 38 through 42, wherein said laser gain medium (1)is a broad emission band laser material suitable for femtosecond lasergeneration.
 44. The solid state laser according to one of the claims 38through 43, further comprising a second curved mirror (15) between saidwaist (14) and a second end of said cavity.
 45. The solid state laseraccording to claim 44, wherein a distance between said second curvedmirror (15) and said second end is on the order of 40 centimeters. 46.The solid state laser according to claim 44 or 45, further comprising aprism pair (18, 18′) between said second curved mirror (15) and saidsecond end of said cavity for group velocity dispersion compensation.47. The solid state laser according to one of the claims 38 through 46,wherein said plurality of layers (43) is on the order of 30 in number.48. The solid state laser according to one of the claims 38 through 47,wherein each of said plurality of layers (43) has a thicknessrespectively of approximately 72.3 nanometers and approximately 88nanometers.
 49. The solid state laser according to one of the claims 38through 48, wherein a total optical thickness of said structure (44)corresponds to half a wavelength.
 50. The solid state laser according toone of the claims 38 through 49, wherein said dielectric layers (45, 46)are three or more in number.
 51. The solid state laser according to oneof the claims 38 through 50, wherein said dielectric layers (45, 46)have a reversed order in terms of their index of refraction, withrespect to the order of the refractive indexes of the layers underneath,thereby forming a resonant structure.
 52. A solid state laser comprisinga laser gain medium (1), pumping means for pumping said laser gainmedium (1), a laser cavity, and a beam influencing system (B) with asemiconductor saturable absorber mirror (17), wherein said beaminfluencing system (B) is located between said laser gain medium (1) anda first end of said laser cavity, said laser gain medium (1) comprisesat least a first face (3) for receiving pumping energy from said pumpingmeans, said first face (3) being made reflective at a laser frequency ofsaid laser, whereby said laser gain medium (1) forms a second end ofsaid laser cavity, and a second face, and said pumping means comprise afirst part with a pumping source (4) and a second part, said firstpumping source (4) producing a first pumping beam at said first face (3)and said second part producing a second pumping beam at said secondface.